The present invention relates to optical systems, and more particularly to a method and system for providing a liquid crystal filter that is insensitive to the polarization of the light.
Polarization insensitive liquid crystal tunable filters are used to filter particular wavelengths of light. FIG. 1 depicts one such conventional polarization insensitive liquid crystal tunable filter system 10. This conventional system 10 includes an input signal 12 provided on a fiber, a single fiber collimator 14, a first polarization beam splitter 16, a first right angle prism 22, a first half wave plate 24, a conventional liquid crystal filter unit 26, a second half wave plate 28, a second right angle prism 30, a second polarization beam splitter 32 and a second single fiber collimator 34 that provides an output signal 36. The first polarization beam splitter 16 splits the input optical signal 12 into two polarization components 18 and 20 with parallel and perpendicular polarization direction relative to the plane of the page, respectively. The first component 18 is transmitted through the first half wave plate 24 with its polarization rotated 90 degree, filtered by the conventional liquid crystal tunable filter unit 26 at region 25, then reflected by the second right angle prism 30 to the second polarization beam splitter 32 and then reflected to the output. Similarly, the second component 20 is reflected by the first right angle prism 22, filtered by the conventional liquid crystal tunable filter unit 26 at region 27 and then transmitted through the second half wave plate 28 with its polarization rotated 90 degree. Then the second component 20 is provided to and transmitted by the second polarization beam splitter 32. Thus, the first component 18 and the second component 20 are recombined to a single optical signal and output as conventional filtered optical signal 36 through single fiber collimator 34.
FIGS. 2A and 2B depict front and side views, respectively, of the conventional liquid crystal tunable filter unit 26, which includes two glass slides 48A and 48B two transparent electrodes 44A and 44B, high reflective coatings 40A and 40B, liquid crystal 46 and spacers 42A and 42B. The two spacers 42A and 42B set the distance between the high reflective coatings 40A and 40B. The elliptical shaped liquid crystal molecules 47 are aligned in the vertical direction and can be oriented clockwise or counter clockwise with the voltage applied on the electrode 44A and 44B. Since the polarization direction of first component 18 and second component 20 are aligned with that of liquid crystal at regions 25 and 27, both components 18 and 20 experience a refractive index or optical phase delay change with the changes of voltage applied on the electrode 44A and 44B. The optical phase delay change for both components 18 and 20 provides center wavelength tuning on the combined optical signal 36.
Although the conventional liquid crystal tunable filter system 10 functions, one of ordinary skill in the art will readily recognize that there are drawbacks. In particular, the conventional liquid crystal tunable filter system 10 has a polarization separation in its transmitted spectrum. Because of the glass slides 48A and 48B are not perfectly parallel, the thickness of the liquid crystal 46 varies across the length of the conventional liquid crystal tunable filter unit 26. In addition, the density of the liquid crystal 46 may vary-across the length of the liquid crystal tunable filter. Furthermore, as depicted in FIG. 1, the first component 18 and the second component 20 pass through the conventional liquid crystal tunable filter unit 26 at different positions 25 and 27. As a result, the first component 18 and the second component 20 can experience different thicknesses and densities of liquid crystal that introduce a difference on the optical phase delay of the components 18 and 20.
FIG. 3 is a graph 50 depicting the spectra 52, 54 and 56, respectively, for the components 18 and 20 and the output signal 34. The difference in their optical phase delay makes the optical spectra 52 and 54 of the transmitted first component 18 and the second component 20, respectively, separated and the optical spectrum 56 of the output signal 36 broadened with a higher insertion loss. This optical spectrum separation between first and second component 18 and 20 is called xe2x80x9cpolarization spectrum separationxe2x80x9d, which makes the conventional liquid crystal tunable filter system 10 have the following the drawbacks: a relatively high insertion loss, an increased polarization dependent loss and a broadened width for the pass band.
Accordingly, what is needed is a system and method for providing a liquid crystal filter system that is insensitive to the polarization of light. The present invention addresses such a need.
The present invention provides a method and system for filtering an optical signal. The method and system comprise providing a polarization beam splitter, first and second rotators, a liquid crystal tunable filter unit, and first and second prisms. The polarization beam splitter splits the optical signal into a first component having a first polarization and a second component having a second polarization. The first rotator rotates a polarization 45 degrees in a first direction, receives the first component from the beam splitter and is located between the liquid crystal tunable filter unit and the polarization beam splitter. The liquid crystal molecules of the liquid crystal tunable filter unit are aligned 45 degrees relative to a vertical direction. The liquid crystal tunable filter unit receives the first component from the first rotator, provides the second component to the first rotator and is between the first rotator and the second rotator. The second rotator rotates the polarization 45 degrees opposite to the first direction, receives the first component from the first rotator and provides the second component to the liquid crystal tunable filter unit. The first prism is located in proximity to the second rotator, receives the first component from the second rotator and provides the second component to the second rotator. The second prism is in proximity to the beam splitter, receives the first component from the first prism and provides the second component from the first beam splitter to the first prism.
According to the system and method disclosed herein, the present invention provides a liquid crystal tunable filter system that is insensitive to the polarization of the input signal.